The present invention relates to a noise canceler for a video signal, and more particularly to a noise canceler which cancels the noise of a video signal using a correlation between horizontal lines.
Generally, video signal processing systems, e.g., televisions, VTRs, etc. include circuits for canceling the noise contained in a video signal, as means for acquiring good picture quality.
FIG. 1 is a block diagram of a conventional noise canceler for a video signal. The conventional noise canceler comprises a high-pass filter (HPF) 3 for high-pass-filtering a video signal input via an input port 1, a limiter 4 for extracting and outputting the noise components below a threshold from the output of HPF 3, and a subtracter 2, for subtracting the output of limiter 4 from the video signal and outputting the result via an output port 5.
FIGS. 2A-2D are operation waveforms of the conventional noise canceler shown in FIG. 1, with N indicating noise and TH representing the threshold of the limiter.
FIG. 2A shows a video signal input via input port 1 and containing noise. FIG. 2B shows the video signal having passed through HPF 3, whereby a pulse is formed in correspondence with the sharply rising leading edge thereof. FIG. 2C shows noise components in which the signal of FIG. 2B is limited to a predetermined threshold level. FIG. 2D shows the output in which the noise components of FIG. 2C is subtracted from the video signal of FIG. 2A. In FIG. 2D, however, it is noted that the noise components are not canceled in the portions in which the signal is sharply changed.
Referring to FIGS. 1 and 2A-2D, the video signal of FIG. 2A is input to subtracter 2 and HPF 3 via input port 1. Limiter 4 is connected in series to HPF 3 and extracts and outputs to subtracter 2 the noise components contained in the video signal. Subtracter 2 cancels the output of limiter 4, that is, the noise components of FIG. 2C, from the input video signal, and then outputs the video signal without noise to the output port 5. The noise canceler of FIG. 1 can cancel most of the noise components. i.e., low-level components, but is unable to cancel those above the threshold level. As shown in FIG. 2C, since, at the transitional edges of a signal, the noise occurs at a level higher than the threshold, the noise is blocked by limiter 4 and thus cannot be canceled.
FIG. 3 shows another conventional noise canceler for a video signal which cancels noise using a correlation of the horizontal lines of a video signal.
Referring to FIG. 3, a video signal is input to first and second subtracters 12 and 13 via input port 11. The output signal of first subtracter 12 is delayed by delay line 14 by 1 H and is input to second subtracter 13. Here, H denotes a horizontal scan period. Second subtracter 13 subtracts the delayed signal input frown delay line 14 from the video signal input from input port 11 so as to extract and output the noise components and a correlation component to limiter 15. Limiter 15 extracts the noise components and sends them to first subtracter 12. First subtracter 12 subtracts the noise components from the video signal, and feeds back the subtracted video signal to delay line 14 so that the video signal is averaged to cancel the noise and is output to output port 16.
However, in the circuit shown in FIG. 3, if there is no correlation between horizontal lines, other signal components (besides the noise components) of the video signal are canceled. This deteriorates the resolution of a reproduced picture.
U.S. Pat. No. 4.885,639 (Dec. 5, 1989, by Sony Corporation of Japan, entitled "Apparatus for Eliminating a Noise Signal") has been presented to overcome the drawbacks of the conventional noise cancelers of FIGS. 1 and 3. In the above patent, an A/D converter, a limiter having multiple threshold values with respect to a digital signal and a switching circuit for selecting the outputs are installed at the output port of the conventional circuit shown in FIG. 1 to cancel noises at the edges. Further, a window detector and a correlation detector are provided at the output port of the HPF of the conventional circuit, so as to solve the disadvantages which exist when there is no correlation between lines. However, the circuit in the above patent is very complicated.